Electrically heasted bedcover and switch therefor



ELECTRICALLY HEATED BEDCOVER AND SWITCH THEREFOR Filed Nov. 30, 1966 h v 'I/IKIIIIII! 2.7 @3 120' '9' INVENTOR 2 F EDWW E. MILLS mmm /g mw/wz ATTORNEYS United States Patent 3,408,469 ELECTRICALLY HEATED BEDCOVER AND SWITCH THEREFOR Edwin R. Mills, Raleigh, N .C., assignor to Fieldcrest Mills, Inc., Spray, N.C., a corporation of Delaware Filed Nov. 30, 1966, Ser. No. 597,960 4 Claims. (Cl. 337-94) ABSTRACT OF THE DISCLOSURE A malleable calibration component is provided in a thermostatic switch for an electrically heated bedcover, to cooperate with a bimetallic strip which is one member of an electrical switch couple. The malleable calibration component, preferably in strip form, biases the bimetallic strip for contact with a fixed contact point within an elongate casing while permitting the bimetallic strip to disengage the fixed contact point at a predetermined elevated temperature, and may be selectively deformed to attain narrow calibration tolerances and permit recalibration of the switch.

This invention relates to an electrically heated bedcover and a thermostatic switch therefor and, more particularly, to such a bedcover wherein an overheat protective means is provided in the bedcover and includes a plurality of thermostatic switches which are readily calibrated to assure proper operation in the event an overheated condition occurs.

Electrically heated bedcovers, such as electric blankets, commonly comprise a textile fabric base or shell, an electric heating circuit including a resistance heating wire disposed in channels formed in the fabric shell, and an overheat protective means for removing electric power from the heating circuit upon the occurrence of a localized overheated condition. An overheat protective means is considered necessary, and is required by Underwriters Laboratories for approval of a bedcover, due to the possible occurrence of a localized overheated condition in the blanket and the resulting dangers of fire and injury to a user.

Such a localized overheated condition usually occurs due to folding or overlapping of the bedcovering or due to the positioning thereon of articles such as clothing or other blankets, with resulting entrapment of heat generated in the resistance wire.

In order to protect against the continuance of a localized overheated condition, it is conventional for the overheat protective means of an electric blanket to include a plurality of temperature sensitive devices, such as inexpensive thermostatic switches, which operate to control the application of current to the electrical heating circuit. Where inexpensive thermostatic switches, relying on bimetallic strip elements for switch actuation, are used as the overheat protection means, each switch must be carefully calibrated in order to assure proper operation thereof.

In order to obtain acceptable performance for the large number of such switches so used, switch manufacturers have heretofore exerted particular care in maintaining close assembly tolerances for the switches, in order to meet the necessary calibration tolerances. Blanket manufacturers are aware that the calibration of such switches as obtained from switch manufacturers is not maintained when a switch is actuated a number of times in response to overheated conditions. Instead, the temperature at which the switch will open progressively changes with each successive occurrence of an overheated condition, and the switch eventually becomes so far out of calibra- 3,408,469 Patented Oct. 29, 1968 tion that satisfactory operation of the bedcovering is impossible.

Heretofore, the loss of calibration of a thermostatic switch in an electric blanket has resulted in the switch opening at progressively lower temperatures until it opens at the normal operating temperature of the heating circuit, and the blanket is no longer electrically heated under any circumstance. When this occurs, the blanket is frequently either returned to the manufacturer for replacement under a warranty or returned for repair. Conventional practice upon such return has heretofore been to strip the electrical circuit from the blanket and entirely replace that circuit with new parts, including new thermostatic switches, inasmuch as the recalibration of a defective switch has not been feasible.

With these difliculties in mind, it has been determined that both the initial manufacture of electrically heated bedcovers and the repair of blankets which have been subjected to repeated occurrences of an overheated condition would be facilitated were a thermostatic switch available which would facilitate both initial calibration and subsequent recalibration. Desirably, such a thermostatic switch would permit accommodation of wide assembly tolerances, while facilitating the attainment of narrow calibration tolerances, thus permitting a switch manufacturer to expend less care in controlling the initial assembly of parts to form the switch while maintaining a satisfactory statistical average operating temperature for the production run of switches. Further, such a switch should desirably be easily recalibrated should calibration be lost as a result of repeated actuation.

It is accordingly an object of the present invention to provide an electrically heated bedcover having an overheat protective means including a plurality of thermostatic switches having the desirable characteristics described above. In accordance with this invention, this object is realized by the provision, in a thermostatic switch, of a means for determining the position of a portion of a bimetallic strip, which means is not subject to thermal distortion and is readily bendable to permit attainment of a desired calibration.

A more specific object of this invention is to provide, in a thermostatic switch, a malleable calibration component, preferably as a strip contiguous with and extending along a predetermined portion of 'a bimetallic strip, so as to control the position of a portion of a bimetallic strip and thus obtain the desirable characteristics enumerated above.

Some of the objects and advantages of the invention having been stated, others will appear as the description proceeds, when taken in connection with the accompanying drawings, in which:

FIGURE 1 is a perspective view of an electrically heated bedcover in accordance with this invention;

FIGURE 2 is a schematic illustration of a portion of the bedcover of FIGURE 1;

FIGURE 3 is a perspective view, partly broken away, of one form of thermostatic switch in accordance with this invention, as prepared for calibration;

FIGURE 4 is a view similar to FIGURE 3 of the switch as prepared for use in the blanket of FIGURE 1;

FIGURE 5 is an elevation view, in section, of the switch of FIGURE 3, taken substantially as indicated by the arrows 5-5 in that figure; and

FIGURE 6 is a view similar to FIGURE 5 of another form of thermostatic switch in accordance with this invention.

Referring now more particularly to the drawings, the electrically heated bedcover and thermostatic switch of this invention are there shown, with the bedcover being generally indicated at 10. The bedcover 10 includes a URE 2), an electrical heating circuit including a resistance wire 12 threaded through channels formed in the shell'11, and an overheat protective circuit having a plurality of thermostatic switches generally indicated at 14 distributed throughout channels in the shell 11. One of the-thermostatic switches 14 has been enlarged, shown in FIGURES 3-5, and is described in greater detail hereinafter. Conductors forming portions of the heating circuit and of the overheat protective circuit are brought to the exterior of the blanket shell 11 and terminate at a connector 151A control box 16 is joined to the connector 15, to complete the circuitry involved in the electrically heated bedcover 10, and may be connected to a suitable source of electrical current for energizing that circuitry, as generally known.

In order to facilitate the initial attainment of narrow calibration tolerances and any subsequent recalibration of thethermostatic switches 14 included in the overheat protective circuit of the electrically heated bedcover 10, this invention contemplates that each ofthe thermostatic switches 14 incorporate a novel provision for ready calibration. The manner in which calibration is accomplished and the provision made for ready calibration may be more fully understood in conjunction with a description of the components of one of the switches 14 and the manner in which those components are assembled.

Each of the thermostatic switches 14 comprises a elongate hollow casing 18 (FIGURES 3-5) which is preferably formed with planar interior walls and a generally rectilinear cross-sectional cohfiguration by stamping from a single piece of suitable material such as lightgauge steel sheet. By suitable folding during the stamping operation, the casing 18 may be provided with an extending tang portion 19, a closed end adjacent the tang portion and an open end remote from the tang portion 19. A fixed contact point'20 is positioned within the hollow elongate casing 18, remote from the open' end thereof, and preferably is provided in the form of a rivet. Irrespective of whether or not the contact point 20 is provided by a rivet or is formed directly from the material of the casing 18, the contact point location thus defined is immovably fixed with respect to the casing 18 and is substantially in the plane of the flat interior wall of the elongate hollow casing 18. Where the fixed contact point 20 is provided by a rivet, it is convenient to form an opposing opening 21 in the casing 18, to provide access for seating of the contact point rivet.

Within the casing 18 is a bimetallic strip 22, which provides a temperature responsive switch member. The free end portion of the bimetallic strip 22 (to the right in FIGURE is disposed for temperature responsive movement in co-action with the fixed contact point 20, and forms with that fixed contact point a switch couple adapted for interconnection with an electrical circuit, as best understood by reference to FIGURE 2. Under normal operating temperature conditions, the switch couple In order to provide for calibration of the temperature at'which the free end of the bimetallic strip of 22 moves out of engagement with the fixed contact point 20, the thermostatic switch 14 includes a malleable calibration component, such as a strip 24 extending along the bimetallic strip 22 at a portion thereof remote from the free end. Where the calibration component is a strip 24, that strip is contiguous with the bimetallic strip 22, and

x determines the rela ve'bsitibn of that portion ofthe' bimetallic" strip 22 with respect to the fixed contact point 20 irrespective of any temperature condition. As used herein, the term malleable is intended to indicate a material which is relatively stiff and rigid without being springy, and which may be readily deformed by the application of suitable;forcewithout springing, back ;to the undeformed position; Mildysteel is one material which has suitable characteristics for the purposes here intended, without introducing the-temperature responsive distortions and springinssinherent in 'the'bimetallic'st'rip 22.

The calibration strip 24 and' bimetallic strip 22 are secured in and electricallyinsulated from the casing 18 at the end thereof which is remote from the fixed contact 20. This securment'and electrical insulation may be readily obtained through the meet a pair of blocks-26, 27 of electrically insulating material. The blocks 26, 27 are sized to sandwich the contiguous portions ofthe strips 22,24 and to snugly fit in the open end of the casing 18. Subsequent to insertion of the sandwich of strips 22, 24 and insulating blocks 26, 27, the folded wall of, the casing 18 is crimped inwardly upon the superposedvblocks and strips to secure the assembly together.

Toprovide'for interconnection of the switch couple with the overheat protective circuit, as described above, terminals are provided for receiving and securing electrical conductors such as light-gauge insulated wire. For interconnection of the fixed contact'point 20 with the electrical circuit, afirst terminal 28 is secured to the casing 18, and electrical current flows through the material of the casing 18 to the fixed contact point 20. In one form of the thermostatic switch, 14 of this invention, reliance is placedon the material of the calibration strip 24 for conduction to the bimetallic strip 22 and thus to the free endthereof which co-acts with the fixed contact point ,20 (FIGURES 3f Inaccordance with this form, a terminal portion 29 may be formed integrally with an externally exposed end of the calibration strip 24, extending outwardly of the casing 18. i

Subsequent to connection of the thermostatic switch 14 with conductors for the overheat protective circuit, a watertight flexible sheet material covering or protective sleeve 30 is'positioned enshrouding the casing 18 and the first and secondterminals 28, 29 together with the electrical .conductorsconnected thereto. The sleeve 30 preferably is of a suitable .plastic materiahsuch as vinyl,

which may be heat sealed in order to assure protection of the otherwise exposed electrically .conductive portions of the thermostatic switch'14, as is necessary when the bedcover 10 is vwashed. f'" v i In preparing theoverheat protective circuit for use in bedcover 10, each of the thermostatic switches 14 is calibrated for switch operation under predetermined thermal conditions. 'In performing such calibration, the assembled switch is brought to the predetermined calibration temperature and a suitable instrument (such as the punch 31.indic'ated in' phantom lines in FIGURE 5) is inserted through the appropriate one of'a 'pair of aligned, opposing calibration openings 32, 33 formed in the casing 18.' The calibration openings 32, 33 are substantially aligned with the free end of the calibration strip 24, in contiguous relation with the bimetal strip 22, so that-the strips may be bent from the fixed'end of their securement, where they are sandwiched between the insulating blocks 26, 27.

Once each switch 14 has been calibrated by bringing the free end of the bimetallic strip 22 into engagement with the fixed contact point 20 while the switch is under the desired thermal conditions, the active length of the bimetallic strip 22 for temperaturer'esponsive movement is' determined by the distance from the free end. of the calibration strip 24 to the free end of the bimetallic strip 22. In order to obtain suitable positioning of the strips for calibration purposes, and permit a wide range ofassemblyline tolerances to be found accept'ableby maintaining close calibration tolerances, it is envisioned by this invention that the calibration strip 24 will extend for no more than one-third of the distance between the secured remote end and the free end of the bimetallic strip 22.

In accordance with this invention, it is occasionally preferably to provide more direct interconnection of a bimetallic strip to the electrical circuit within which a thermostatic switch is to be employed, for purposes of reducing the interface resistance. Where this is preferable, a modified form of switch, indicated generally as 14' (FIGURE 6) may be used. The switch 14' is 'basically identical to the switch illustrated in FIGURES 3-5 and described in greater detail hereinabove, and portions of the switch 14' identical to similar portions of the switch 14 are identified by similar prime numerals. The point of distinction lies in the bimetallic strip 22, which has a free end projectng outwardly of the casing 18', to which a terminal 29' is riveted for connection to an electrical conductor.

In the drawings and specification, there have been set forth preferred embodiments of the invention, and although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention being defined in the claims.

I claim:

1. A thermostatic switch wherein the attainment of narrow calibration tolerances and recalibration of the switch subsequent to repeated occurrences of an overheated condition is facilitated comprising:

an elongate casing with a fixed contact point therewithin,

a bimetallic strip extending within said casing with a free end portion thereof in electrical contact with said fixed contact point, said free end portion being disposed for temperature responsive movement out of contact with said fixed contact point,

a malleable calibration strip extending along a surface of said bimetallic strip remote from said free end thereof and remote from said fixed contact and cooperating with said bimetallic strip to bias the same with the free end thereof in contact with said fixed contact point while permitting said free end to disengage said fixed contact point at a predetermined elevated temperature,

said casing having a pair of aligned opposing calibration access openings therein for insertion of means for deforming said malleable strip so as to calibrate the temperature dependent location of said free end of said bimetallic strip, and

mounting means securing said bimetallic strip and said calibration strip in and electrically insulating the same from said casing at an end thereof remote from said fixed contact point.

2. A switch according to claim 1 wherein said malleable strip extends for no more than one-third of the length of said bimetallic strip and further comprising a first terminal connected to said casing for interconnection of said fixed contact with an electrical circuit.

3. A switch according to claim 2 wherein said calibration strip is of an electrically conductive material and further comprising a second terminal, said second terminal being connected to said malleable strip for interconnection of said free end with an electrical circuit and a protective sleeve enshrouding said casing and said terminals.

4. A switch according to claim 2 further comprising a second terminal, said second terminal being connected to said bimetallic strip for interconnection of said free end with the electrical circuit and a protective sleeve enshrouding said casing and said terminals.

References Cited UNITED STATES PATENTS 2,338,474 1/1944 Wilson 200122 X 2,675,445 4/1954 Youhouse 200--139 2,792,474 5/1957 Dales 200l38 2,820,870 1/1958 Moksu 200l38 2,871,315 1/1959 Dales 200l38 2,961,526 11/1960 Dykes 219212 X 3,119,926 1/1964 Mills et a1. 219-212 3,148,258 9/1964 Dales 200--138 FOREIGN PATENTS 428,874 5/1931 GreatBritain.

BERNARD A. GILHEANY, Primary Examiner. VOLODYMYR Y. MAYEWSKY, Assistant Examiner. 

